Autocatalysis: The self-induced growth of martensitic phase transformations in ceramics

The phenomenon of autocatalysis, wherein a region of elastic phase transformation creates stresses sufficient to drive alone further transformations, is studied for two specific configurations: a rectangular strip and an infinite row of equally spaced circular spots. The phenomenon is associated, not exclusively, with dilatational strains triggered by critical levels of maximum in-plane shear stress. The analysis predicts that there may be a minimum critical transformation stress level below which spontaneous transformation occurs.     

Highly transparent ytterbium doped yttrium lanthanum oxide ceramics

To prepare ytterbium doped lanthania yttria nanopowder a method of laser evaporation of mixed oxides was used. After calcinations of the powder at 1200 °C a pure single-phase solid solution Yb3+:(LaxY1–x)2O3 was formed in the nanoparticles. Influence of lanthanum oxide as an isovalent additive on the yttria structure was investigated. The lanthanium ions were proved to be a good aid to sinter yttria ceramics doped with Yb3+ at moderate temperatures about 1650 °С. The ceramics with relative density higher than 99.99% and grain size about 40 μm were fabricated. Full transmittance of 1.8 mm thick Yb0.11La0.23Y1.66O3 ceramics reached 82.5% at 800 nm. This material could be a good gain medium for ytterbium high power pulse lasers.     

Isothermal studies of bainitic and martensitic transformations in some low alloy steels

Ohne Zusammenfassung     

Oxidation of the silicon nitride based ceramics containing oxide additives

Translated from Poroshkovaya Metallurgiya, No. 1 (325), pp. 72–75, January, 1990.     

High resolution electron microscopy investigations of interface and other structure defects in some ceramics

To evaluate occurrence of oxidative stress in circulating blood, we developed standard methods to assess (1) granulocytes status as a source of reactive oxygen species (ROS) and (2) lipid peroxidation (LPO). A simplified and highly sensitive assay was developed by utilizing the chemiluminescence (CL) from luminol oxidized by ROS. 1. The CL, from 300 microliters medium containing 1% blood, 10 micrograms/ml luminol and 0.025 microgram/ml phorbol myristate acetate, well reflected the primed granulocyte status induced by in vitro contact with lipopolysaccharide (LPS). This CL was weakened slightly by superoxide dismutase and catalase, but markedly decreased by sodium azide. 2. We determined the optimal conditions for the t-butyl hydroperoxide (t-BuOOH)-stimulated CL method to evaluate plasma LPO in experiments on rat plasma added with phosphatidylethanolamine hydroperoxide (PEOOH). The CL from 300 microliters medium containing 6.67% plasma. 10 micrograms/ml luminol and 5 mumol/ml t-BuOOH was proportional to the added PEOOH amount. The integrated CL of the plasma with 0-60 nmol of PEOOH gave values of 8.280-14.213 x 10(6) counts/60 min/tube. 3. Only 100 microliters of freshly drawn blood was enough for the two CL methods to detect the generation of ROS and the occurrence of LPO. These CL methods enabled the determination of the time course of oxidative stress occurrence in circulating blood of rats treated with 5 mg/kg LPS, i.p.     

稀土氧化物对碳化硼陶瓷性能的影响

以稀土氧化物为主要烧结助剂,以碳化硼粉末为基体,采用真空热压烧结技术制备出碳化硼陶瓷.研究了成分配比、烧结工艺对材料致密度及力学性能的影响;分析了稀土氧化物对烧结温度及材料性能的影响,并确定最佳烧结温度;探讨不同添加剂对碳化硼陶瓷显微结构影响及烧结机理.结果表明,以稀土氧化物为主要烧结助剂,其烧结温度降低约80℃;碳化硼陶瓷的最佳材料配方与烧结工艺为:m(B4C):m(La2O3):m(Al2O3):m(C)＝70:6:12:12,烧结温度1 850℃,压力20MPa,保温时间1h;所得碳化硼陶瓷性能:相对密度92.5%,抗弯强度156.76MPa,硬度97HRA;分别以氧化铝和活性碳、氧化钇、氧化镧、氧化钇和氧化镧为烧结助剂时,碳化硼陶瓷烧结过程中形成的新相分别为Al8B4C7、Y3Al5O12、LaAlO3、(Y3Al5O12 LaAlO3).其中含稀土相,尤其是新相LaAlO3与碳化硼颗粒表面有良好的结合,因此提高了致密度,降低了烧结温度.     

稀土氧化物复合ZrO2陶瓷的制备及应用研究进展

稀土氧化物可作为稳定剂、烧结助剂、掺杂改性剂加入到ZrO₂陶瓷材料中,能极大地提高和改善陶瓷材料的强度、韧性,降低其烧结温度,减少生产成本.文中简要综述了稀土氧化物ZrO₂陶瓷材料的制备及应用研究状况,包括Y₂O₃复合ZrO₂陶瓷粉体、富铈稀土氧化物复合ZrO₂陶瓷粉末、Nd₂O₃复合ZrO₂陶瓷材料、Pr₂O₃\Pr₆O₁₁复合ZrO₂陶瓷、La₂O₃复合ZrO₂陶瓷、Yb₂O₃复合ZrO₂陶瓷、Sm₂O₃复合ZrO₂陶瓷材料及氧化锆中掺杂多种稀土氧化物陶瓷粉体的制备和应用,分析讨论了一些需要解决的问题,并展望了稀土复合ZrO₂陶瓷制备技术及未来研究发展趋势.      

Influence of sintering conditions on some properties of metal-aluminum oxide cermets

Summary A study was made of the sintering and properties of materials in the system corundum-metal. It was confirmed that the density of these cermets is influenced by the difference between the sintering temperatures of their oxide and metal constituents. The addition of MgO to the Al₂0₃ was found to have a beneficial effect on sintering in the system corundum-tungsten (molybdenum).Translated from Poroshkovaya Metallurgiya, No. 6 (66), pp. 34–40, June, 1968.     

Operational nucleation in martensitic transformations

Operational nucleation in martensitic transformations is accessible to measurement and governs the reaction kinetics. In essence, it involves the growth start-up of preexisting embryos, and is not concerned with the origin of such embryos. Another important feature is the fast chain-of-events triggered-off after the growth is initiated, leading to the sudden formation of observable martensitic plates. The model of operational nucleation and subsequent growth presented here provides a plausible explanation of the fine-scale phenomena, and permits calculation of the aspect ratio (or growth path) adopted by the advancing particle. E. C. Bain had a long-standing interest in the nature of martensitic nucleation. By strange coincidence, this was the subject of my last conversation with him.     

Cavitation-erosion-lnduced phase transformations in alloys

Previously reported surface phase transformations occurring during cavitation erosion in cobalt- and iron-base alloys have been confirmed. However, an examination of the effect of aging treatments on Stellite 6B failed to demonstrate any simple relationship between transformation characteristics and erosion resistance. For several iron-base transformable alloys, the erosion resistance was shown to be related to strength and again no direct correlation could be established between transformation and erosion. It is concluded that the phase transformation is incidental to the erosion process. However, since many erosion resistant alloys do undergo a deformation-induced phase transformation, a more general explanation for the erosion resistance of alloys with close-packed structures is offered in terms of stacking fault energy.     

Phase transformations in high polymers

Phase transformations in high polymers are reviewed broadly and with the purpose of high-lighting the similarities and dissimilarities between behavior in high polymers and in metals. Emphasis is placed upon the kinetic restraints that make phase transformations in high polymers sluggish for the most part, and upon the molecular factors that underlie them. The review deals specifically with transformations of four kinds : a) transformations between crystallographic polymorphs, b) transformations that accompany deformation, c) phase separations reminiscent of spinodal decomposition, and d) crystallization. In the last example, a discussion of polymer chain folding is given in simple physical terms. This paper is based on a presentation made at a symposium on “Phase Transformations in Less Common Metals: A Dialogue,” held at the Fall Meeting in Cleveland on October 16, 1972, under the sponsorship of the Phase Transformations Activity, Materials Science Division, American Society for Metals.     

Isothermal transformations in iron-chromium-carbon alloys

This paper describes the structures produced in isothermally transformed Fe−Cr−C alloys. It is shown that both the ferrite morphologies and associated carbide dispersions can be related to the relevant growth mechanisms. Preliminary kinetic data suggests that the transformation is controlled by the redistribution of solute within the advancing interface.     

Capillary-infiltrated conductors in ceramics

The infiltration of high conductivity metals into capillaries in ceramic structures have been reported by Schwartz and Wilcox. Others have used the infiltration concept for the fabrication of composite structures for a variety of reasons, thermal expansions, machinability of hard metals, and so forth. In electronic applications, that is the fabrication of ceramic structures for support and interconnection of devices, the reasons for infiltration are evident. They provide highly conductive paths in a controlled manner for dimensions and configurations which approach those of device structures. Experiments were conducted to investigate the wetting and spreading of copper on molybdenum metallized alumina slips. Various surface-active and viscosity-active metals were added to the copper, and ambient conditions were varied. The results indicate that the primary controlling parameters for wetting are the ambient conditions. The infiltrations of copper into porous molybdenum lines were observed by X-radiographs of experimental substrates after increments of time at 1120°C. The substrates were moved from an average temperature of 1040°C into a hot zone at 1120°C, then after an incremental soak time were returned to 1040°C, and subsequently cooled for observation. The penetrations of copper are clearly seen on the X-radiographs so that the process of infiltration may be closely monitored. The results indicate that reliable infiltrations at predictable rates may be obtained. Thus, the technique is a reliable means of achieving high conductivity metals within a ceramic substrate. There is no indication of a limitation on penetration distance on well formed lines as small as 5 mil wide. Electrical conductivity of infiltrated lines was observed to follow the volume relationship: Σ=v_CuΣ_Cu + v_MoΣ_Mo where Σ = conductivity andv = volume proportion.